It's hard when even all my friends fornicate (hookup sexually) but don't care, even my Christian friends.
If one of your friends claimed to be be a race car driver, would that make him one? Or would you not believe him until you actually saw him driving in a race?
In the same way, claiming to be a Christian is just empty words until a person actually lives by Christ's laws. "You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Therefore by their fruits you will know them" (Matthew 7:16-20).
Your friends who claim to be Christians while having sex outside of marriage are simply lying to themselves and to the world around them. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10). There are going to be a lot of people shocked when they find out that Jesus actually meant what he said: "Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven. Many will say to Me in that day, 'Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?' And then I will declare to them, 'I never knew you; depart from Me, you who practice lawlessness!'" (Matthew 7:21-23).